The role of notching damage on the fracture parameters of ethylene-propylene block copolymers

The influence of the notching procedure on the fracture toughness measured via Elastic-Plastic Fracture Mechanics has been analyzed on four different ethylene-propylene block copolymers with two distinct dimensions, paying special attention to the morphology of the area surrounding the crack tip front. Two sharpening techniques were evaluated: the traditional steel razor blade and the femtolaser ablation process. The fracture toughness of the razor blade sharpened samples was always higher than that of the femtolaser sharpened specimens. Also, the fracture toughness of the razor blade samples was dependent on the thickness of the samples, whereas the fracture toughness of the femtolaser sharpened specimens was not influenced by the dimensions of the test specimens. The microscopic analysis of non-tested samples showed that the crack tip radii were similar for both type of sharpened samples but the damage and its extension ahead of the crack tip was dependent on the notching technique, the copolymer type and the dimensions of the analyzed specimen. The femtolaser sharpened samples presented a very tiny heat affected zone ahead of the crack tip, the size of which was independent of the copolymer type and the dimensions of the test specimen. On the other hand, the steel razor blade sharpened samples showed an area surrounding the crack tip formed by plastic deformation, the length of which increased for the smaller size of sample and for higher ethylene content in the copolymer.